1. Field of the Invention
The current invention relates to a process and device for vulcanizing tires.
2. Discussion of Background Information
In general, the standard process for vulcanizing tires, in particular green tires, independently of the type of curing press used, is carried out by means of an expandable bladder, which preshapes the tire to be vulcanized and presses it against the curing mold.
The bladder fulfills a series of functions, namely centering the blank as long as the mold is not closed, separating the heating medium from the blank, shaping the bead toes and the inner-liner of the tire, and sealing the system in relation to the outside.
It has been known for a long time that the process of bladder vulcanization is involves a number of disadvantages. The inflated outline of the bladder deviates sharply from the contour of the blank, particularly in the region of the shoulder of the tire. As a result, relatively large air pockets are produced between the bladder and tire, which have to be eliminated during the molding process. For this reason, air removal grooves are provided on the outside of the bladder. However, these air removal grooves admit inner plate material of the tire and as a result, diminish the layer thickness.
Furthermore, the bladder comes to rest in the blank, in the crown and bead regions first and in the shoulder regions of the tire last. As a result, an air transport takes place inside the blank along the belt cord strands toward the belt edges. The air collected there must be absorbed by the shoulder components and weakens the vulcanization capacity in the milling zone region. Due to a smoothing effect during the shaping of the inner-liner, tire material is transported in the direction of all component impacts, which can lead, for example, to displacements of the fiber distribution in the casing. This can lead to bumps and indentations in the side wall of the finished, pumped tire.
Since in general, one bladder is used for several tire dimensions, this can also cause air removal problems between the bladder and green tire.
The quality of the finished tire that has been cured with bladders can be impaired because bladders are used which inflate more on one side and consequently de-center the tire in the press, and furthermore, because bladder defects, for example holes, can appear during vulcanization.
After press shutdowns, the bladder must be manually preshaped before reloading, which frequently leads to quality problems when starting up, due to the lack of an automatic time/temperature process.
In addition, because of its wall thickness, the bladder restricts the heat flux between the heating medium and the tire, which adds to the costs of curing time and energy balancing.
Furthermore, the bladder requires an internal treatment of the green tire, for example the injection with silicon-containing solution that is either benzenous or aqueous. This solution hampers welding on all tire components on the inside and is also damaging to the environment.
In addition, the bladder manufacturing and bladder disposal costs represent a considerable economic factor.
The structural optimization of tires is carried out by means of assessing radial and circumference sections and micro-sections of the vulcanized tire. This constitutes a two dimensional assessment method. Structural material distribution weaknesses which come to light in a three dimensional fashion on the inside of the tire as a result of a pressure of the medium that acts equally in all directions, are smoothed by the bladder and can no longer be detected. The result can be hidden weak points or welding weaknesses or quality-impairing material displacements in the internal component structure of the tire, which are only possible to see, if at all, as bubbles or creases from the shifting of material.
Various devices have already been proposed for the bladderless vulcanization of green tires. A device of this kind has been disclosed, for example, by DE-A 2228504, in which a clamping device for the tire bead is provided in the tire mold and has a press device for the tire bead. Essentially, the inner chamber of the green tire is intended to be sealed through the use of press devices of this kind. A realization of systems of this kind has failed primarily because it is much too expensive to build and is susceptible to malfunction.